The Silica Gel Surface and Its Interactions with Solvent and Solute in Liquid Chromatography

The multilayer formation of water on silica gel is discussed, and experiments indicating the existence of three layers of water are described. The interaction of active silica gel with solvent is also considered. Activated silica gel as used in chromatography, appears to contain one strongly hydrogen bonded water molecule per silanol group which can dispersively interact with nonpolar solvents forming a monolayer. In contact with a polar solvent, however, the hydrated silanol group can hydrogen bond to the polar solvent and form a strongly held solvent layer on top of which a bilayer of polar solvent can form by polar interactions with the first layer. The mechanism of solute interaction with activated silica can be described as follows. Solutes interact with the multilayer surface in a chromatographic column in two ways. If the solvent layer is weakly held by dispersive forces as in the case of a nonpolar solvent, the solute can displace the solvent layer and interact directly with the hydrated hydroxyl groups. If, on the other hand, the solvent is polar and is strongly held by hydrogen bonding forces to the hydrated silanol group, solutes associate directly with the polar solvent layer but do not displace it unless the solute has a polarity similar to the solvent and is consequently eluted at a high k’ value.